The present invention relates to a control device for an automatic transmission in a vehicle capable of automatically shifting a gear of the automatic transmission to a gear at which the vehicle can travel at the lowest fuel consumption. More particularly, the present invention relates to a technology of performing a shift control for reducing a fuel consumption amount without degrading traveling performance, in an automatic transmission in a vehicle equipped with a variable cylinder engine that can be operated in a cylinder deactivation state.
As a control device for an automatic transmission in a vehicle, there is a control device in which a plurality of types of shift maps that define shift characteristics are prepared, any one of the shift maps is selected based on a determination of whether a vehicle is traveling on an uphill road or a downhill road, a gear (also referred to as a shift position) is determined according to a traveling speed (vehicle speed) of the vehicle and an accelerator pedal opening degree involved in an accelerator operation of a driver, based on the selected shift map, and a transmission is automatically shifted to the determined gear (see Japanese Patent No. 2959938). For example, shift-down control or shift-up control is performed based on the previously prepared shift map for an uphill road while a vehicle travels on an uphill road.
Alternatively, as a control device for the automatic transmission in a vehicle, there is a control device that performs shift control referred to as a low fuel consumption mode and the like, and performs automatic shift from a gear of the transmission to a gear which a vehicle can travel at the lowest fuel consumption within a range in which a vehicle speed is not reduced (see Japanese Patent No. 4696692). By doing so, for example, even after the shift-down is made while a vehicle travels on an uphill road, a shift-up control may be performed quickly so as to reduce the fuel consumption amount of an engine. However, when the shift-up control is made quickly while a vehicle travels on an uphill road, a driving force may be insufficient and a stepped amount of an accelerator pedal by a driver may suddenly increase, and in the worst case, for example, another shift-down may cause an increase in fuel consumption. Therefore, if an allowance driving force is less than a predetermined required allowance driving force when being shifted up to the next gear, the shift-up to the next gear is prohibited, that is, the shift-up shift control is not performed.
As an engine equipped in a vehicle, a variable cylinder engine that can be operated in a cylinder deactivation state has been known. In order to reduce an amount of fuel consumption, the variable cylinder engine is configured to switch an operation state of the engine between a full-cylinder operation in which all the cylinders are operated and a partial cylinder deactivation operation (a cylinder-resting operation) in which some of the cylinders are deactivated (see Japanese Patent Application Laid-Open No. 2006-292114).
As described above, in view of the reduction in the fuel consumption amount of the engine, a gear shift control for a vehicle traveling on an uphill road may be performed as a shift-down control at first, and then, once sufficient driving force has been secured after the shift-down, a shift-up control should be performed as soon as possible. However, in the case of a vehicle equipped with the variable cylinder engine, there is a possibility that any cylinder deactivation operation of the engine cannot be operated due to the performance of the shift-up control. In this case, if the engine is currently operated in a cylinder deactivation operation, the engine is caused to switch from the current cylinder deactivation operation to the full-cylinder operation due to the shift-up of the gear even though the shift-up control is performed in order to reduce the fuel consumption amount of the engine, so that efficiency of reduction of the fuel consumption amount must be decreased.